1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having electrically writable fuse elements and more particularly to a fuse data read circuit adapted to automatically adjust reading parameters used in reading data from the fuse elements.
2. Description of the Related Art
There are various types of electrically writable fuse elements (hereinafter referred to as fuse elements) which are built in to semiconductor integrated circuits: gate oxide destruction type, deep trench (DT) oxide destruction type, polysilicon blow type, and so on.
With the fuses, there is a difference in resistance between a “1” fuse, which stores “1” data as the result of being previously written with “1”, and a “0” fuse, which stores “0” data without being written with “1”.
To read data from the fuse, it is required to apply a constant voltage across the fuse. After the voltage application, the voltage across the “1” fuse rises faster and consequently reaches a large value more quickly than the voltage across the “0” fuse. After the lapse of a fixed period of time from the voltage application, therefore, a large voltage difference is produced between the “1” and “0” fuses according to their difference in resistance.
In reading data from a fuse, the voltage across the fuse is compared with a decision reference voltage at a point of time after the voltage application to thereby decide whether the fuse is a “1” fuse or a “0” fuse. For such voltage comparison and fuse data decision, a fuse data read circuit is used. With a conventional fuse data read circuit, the read timing or the decision reference voltage for detecting the voltage across the fuse after voltage application is fixed.
As semiconductor integrated circuit processes advance, the difference in resistance between “1” and “0” fuses tends to become more smaller. In addition, there is a problem that all the characteristics of fuses cannot be appreciated at the initial stage of integrated circuit design.
However, when the read timing or the decision reference voltage, which is fixed, is not set suitably, it will be required to redesign circuits that use fuse data.
Japanese Patent Application KOKAI Publication No. 2002-231816 discloses a technique to permanently adjust a trimming circuit containing one or more fuses offline according to the optimum trim bit sequence.